That is not to say they would never have good policies but that fundamentally they would always be left slanted and lean towards union/ lower socioeconomic groups.
That's an amnesiac view. The Labour government elected in 1984 brought in (much needed) economic reforms, and replaced a National government which was behaving more like a hard-core socialist regime, with "think big" state funded projects, price and wage controls etc - as well as a divisive authoritarian manner from the PM.
IMO - over time there's not been consistency from either party. There's perhaps a perception that National is socially conservative, Labour socially liberal, but even that's not consistent. I suspect that many people vote one way or the other based on perception of social policy leaning from "spin" rather than policy/manifesto, but once a party is elected they forget the past.
Labour's biggest problem is that despite spin from National, not a lot has changed. We still have WFF, you're not allowed to thrash your kids, social liberal policies have been enacted anyway (ie same sex marriage), we haven't rounded up unemployed youths and sent them to army camp - then to fight crusades with the US, there's still a minimum wage, and changes to the tax system have been moderate.